The refining of metals from their ores in a very energy intensive process and most metals in common use, e.g. aluminum, zinc, iron and their alloys etc. are thermodynamically unstable with respect to their oxides. Under ambient conditions the oxidation of these metals is called corrosion and the corrosion processes are very dependent on the environment in which the metals are kept. For example, steel objects kept in a clean low humidity environment, such as is found in the southwest United States deserts have very low corrosion rates, whereas the same object placed in a humid marine environment may corrode much more rapidly. In addition to humidity, other factors are known to promote corrosion, including temperature, salts (e.g. the marine environment above) and corrosive gases.
The cost of corrosion is very high, it has been estimated that the cost is equivalent to 3 to 5% of the gross domestic product of industrialized countries or $300 billion for the United States in 1995 {P. R. Roberge “Handbook of Corrosion Engineering” McGraw-Hill, New York, 2000}. From a more practical perspective, many metal objects are used at locations different from where they are manufactured and may sometimes suffer damaging corrosion before even being put into service. Once in service, corrosion may still occur. A whole industry has been developed to measure and control corrosion processes and details can be found in standard texts such as 1) P. R. Roberge “Handbook of Corrosion Engineering” McGraw-Hill, New York, 2000 2) L. L. Sheir, R. A. Jarman, G. T. Burstein “Corrosion, vols. 1 & 2”, 3rd Ed., Butterwork-Heinmann Ltd, Oxford (1994).
One part of the corrosion monitoring arsenal are corrosion sensors, devices that can be placed on a potentially corroding object or in the environment of the object that either provides a real time measure of the corrosion rate or a measure of the propensity of the environment towards causing corrosion. Corrosion occurs in many different environments and so there are many different types of corrosion sensor.
Some of the techniques that have been applied towards developing corrosion sensors include:
a) Electrical resistance—as the sensing element is corroded, its electrical resistance increases providing a measure of the corrosion rate.
b) Inductive resistance—changes in the thickness as the sensing element is measured by changes in the inductive resistance of a coil embedded in the sensor, thus providing a measure of the corrosion rate.
c) Linear polarization resistance—an electrochemical technique in which a small potential perturbation is applied and the resulting current is measured. The slope of the potential vs. current curve is the polarization resistance, which provides a measure of the uniform corrosion rate.d) Electrochemical impedance—EIS is a newer technique that in which a small alternating potential is applied and the resulting current analyzed to provide impedance and phase information. EIS provides a great deal of information about corrosion processes and the status of protective coatings, but the analysis is often complex.e) Electrochemical noise is a newer technique that measures the electrical noise that is associated with some corrosion processes, such as pitting.f) Hydrogen sensors: Hydrogen evolution is one of the two common cathodic processes in corrosion (the other is oxygen reduction) and so the presence of hydrogen gas is indicative that corrosion is occurring.
There are many other techniques for corrosion engineering and these can be found in standard texts such as P. R. Roberge “Handbook of Corrosion Engineering” McGraw-Hill, New York, 2000.
One important area of corrosion monitoring is atmospheric corrosion. It is well know that some environments are more corrosive that others. For example iron objects corrode significantly faster near a marine environment than when located in an inland rural area. Typical atmospheric species that promote corrosion are chloride salts (marine environment) and corrosive gases such as sulfur dioxide (SO2), hydrogen sulfide (H2S), hydrogen chloride (HCl) and oxides of nitrogen (NO2, N2O4, NO) associated with industrial pollution, such as combustion processes from diesel or gasoline powered engines. Typically these compounds promote corrosion by providing an acidic environment, an electrolyte for corrosion cells and/or for dissolving protective oxide layers from metal surfaces.
For many objects it is important that they are not being exposed to a corrosive environment. For example, by the time that corrosion products are seen on an object with a decorative coating, the damage would already be done and it would be too late to apply remedial measures. Therefore it is important to provide early warning that the environment is potentially corrosive.
To this end several atmospheric monitors have been developed. For example Sandia National Laboratories recently developed an atmospheric corrosion monitor that measured the reflectivity of an optically thin metal mirror (10-30 nm) on the end of an optical fiber. As the metal is corroded or reactive species chemisorb, the reflectivity decreases.
Another method that has been developed is the corrosion fuse in which a small wire coil under tension is exposed to the corrosive atmosphere. Corrosion leads to the wire stretching or breaking which can be detected by a fiber optic or other means.
Gas sensors have been used to directly monitor the presence of corrosive gases. Since these sensors detect the gases directly, early warning of a potentially corrosive atmosphere can be provided. However, in most cases the concentrations of the gases are at low or sub part per million concentrations and the cost of commercially available gas sensors and associated instrumentation is prohibitive for many applications. These and some other methods of corrosion monitoring are described in P. R. Roberge “Handbook of Corrosion Engineering” McGraw-Hill, New York, 2000.